US 3917781 A
A method for preparing and molding concrete so as to impart great strength per weight, minimize or otherwise alter shrinkage and creep, reduce porosity and give a superior as-molded surface. Aggregates are dry mixed with cement (or cement-filler mixture) in a preparation such that the bulk volume of dry cement (or cement-filler mixture) is approximately equal to the volume of void space which the aggregates would have if the cement were not present. A mold is filled with the resultant dry mixture, the ingredients of which are so proportioned that a particular geometry of particle contact is achieved upon compaction. The quality and geometry of this contact (coupled with the properties of the particular filler materials) determine the properties of the finished product. Then, with no disturbance to this compacted structure of particle contact, water is injected and distributed by a self-metering process dependent upon capillary action which itself is dependent upon the geometry of particle contact. With the assistance or opposition of gravitational forces and with or without increased pressure differential, this water is introduced in a manner such that the water sweeps the air from the remaining voids and ejects that air into the atmosphere. The resultant wet, compact mixture is then cured in the mold or forms.
Description (OCR text may contain errors)
United States Patent  Gabriel et a1.
45 Nov. 4, 197
Related US. Application Data  Continuation-impart of Ser. No. 886,556, Dec. 19, 1969, which is a continuation-in-part of Ser. No. 792,370, Jan. 21, 1969, abandoned.
 U5. Cl. 264/71; 264/69; 264/333  Int. Cl. B28B 1/08  Field of Search.. 264/69, 71, 72, 240, DIG. 43,
 References Cited UNITED STATES PATENTS 131,561 9/1872 Ransome et a1. 264/D1G. 43
747,193 12/1903 Leet et a1 264/333 X 849,790 4/1907 Jackson 264/340 1,489,979 4/1924 Cahill 264/256 2,382,458 8/1945 Williams et al. 264/333 2,667,679 2/1954 Jackman 264/69 X 3,238,156 3/1966 Kohrn 260/2.5 AK
3,365,315 l/1968 Beck 260/25 AK 3,441,523 4/1969 Dwyer 260/2.5 AK
3,472,798 /1969 Pitchforth 260/2.5 AK
3,510,392 5/1970 DEustachio 260/ AK 3,524,794 8/1970 Jonnes et a1. 260/2.5 AK 3,582,377 6/1971 Hays et a1. 106/97 3,585,157 7/1967 Beck 260/25 AK FOREIGN PATENTS OR APPLICATIONS 1,159,865 12/1973 Germany OTHER PUBLICATIONS Opportunities in Materials, Proceedings of 4th Buhl Int. Conf. on Materials, Pittsburgh; Pa., Nov. 16-18, 1971; pp. 466-477; article by A. A. Johnson et al. IBM Technical Disclosure Bulletin, Vol. 15, No. 2, July 1972, page 515; Polyurethane Foam with Glass Microbeads.
Jarvis Raask A Lightweight Material, British Chemical Engineering, Vol. 15, No. 9; Sept. 1970; pp. 1165-1167.
Bell et al., Vibratory Compacting of Metal and Ceramic Powders, U.S. Dept. Commerce (1953), pp. 11-13, 16, 17, 22 and 23.
Lea et al. The Chemistry of Cement and Concrete, Edward Arnold, London (1956), pp. 317-319, 334 and 335.
Taylor, W. 11., Concrete Technology and Practice, American Els'evier, NY. (1965), pp. -52, 116-123, 129,131-134 and 181-184.
Primary Examiner-Robert F. White Assistant Examiner-Willard E. Hoag Attorney, Agent, or Firm-Owen, Wickersham & Erickson 57 ABSTRACT A method for preparing and molding concrete so as to impart great strength per weight, minimize or otherwise alter shrinkage and creep, reduce porosity and give a superior as-molded surface. Aggregates are dry mixed with cement (or cement-filler mixture) in a preparation such that the bulk volume of dry cement (or cement-filler mixture) is approximately equal to the volume of void space which the aggregates would have if the cement were not present. A mold is filled with the resultant dry mixture, the ingredients of which are soproportioned that a particular geometry of particle contact is achieved upon compaction. The quality and geometry of this contact (coupled with the properties of the particular filler materials) determine the properties of the finished product. Then, with no disturbance tothis compacted structure of particle contact, water is injected and distributed by a selfmetering process dependent upon capillary action which itself is'dependent upon the geometry of particle contact. With the assistance or opposition of gravitational forces and with or without increased pressure differential, this water is introduced in a manner such that the water sweeps the air from the remaining voids and ejects that air into the atmosphere. The resultant wet, compact mixture is then cured in the mold or forms.
8 Claims, 25 Drawing Figures DENSEST POSSIBLE ARRANGEMENT (S.G.=L)
g A LOOSEST POSSIBLE a E 2 10,000 ARRANGEMENT S.G.= 1.40) B0 c a: E w n: 2, 5,000
% VOIDS IN COMPACTED SAND AGGREGATE COMPLETELY FILLED WITH COMPACTED CEMENT SURCHARGE LOADING I CONSTRAINING CYLINDER COLUMN OF SAND COMPACTED TO ITS MOST DENSE ARRAY '3- PLATFORM MOHR ENVELOPE SHEAR o:
F lG 2 STRESS NORMAL STRESS SHEAR STRESS F I G 3 NORMAL STRESS SHEAR STRESS U.S. Patent Nov. 4, 1975 Sheet2 0f8 3,917,781
FIG-5 STRUCTURALLY STRONG PARTICLES BALANC PARTICLE LOADING F I6 50 DUE TO OPER CHOICE OF COMPACTION AND NUMBER OF STRUCTURALLY WEAK PARTICLES DISTRIBUTED COMPRESSIVE LOAD AW///A,AZ///A////,//// .STRUCTURALLY WEAK PARTICLES SHEAR DUE IMPROPER AMOUNT OF COMPONENT l IG 6Q COMPACTION STRUCTURALLY STRONG STRUCTURALLY STRONG w E- T; 7 00C 00 FIG 7g NON-SYMMETRICAL PARTICLE LOADING DUE TO INCLUSION OF TOO FEW STRUCTURALLY WEAK PARTICLES us. Patent N0v.4, 1975 I hee fs 3,917,781
DENSEST POSSIBLE ARRANGEMENT (S.G.=l.65)
LOOSEST POSSIBLE [LI 2 I0,0 3- 00 ARRANGEMENT (s.G. L40) -'z FEES n: 5,000 gI- Om 3435 40 4'4 6'5 v0|0s IN COMPACTED SAND AGGREGATE A COMPLETELY FILLED WITH COMPACTED GEMENT I 3 /a II I- 4000 w MATERIAL 2 3000 3 3'/ I- k 4 8 m i g 3' MATERIAL M (0 gg I000 F lG. 11 O. E o 0 I00 5'0 0 F IG CEMENT IN cEMENT- MICROBALLOON mm 9000 A E 8000 MIXTURE P g 1000 u I '52 5000 IEXTURE 0 3 4000 g2, 3000 g 2000 0 I000 Y o O I I Q I I l 0 l 2 3 4 5 6 1 a 9 I0 CALCIUM CHLORIDE (BY WEIGHT OF WATER) P BEAM FlG 12 77437 9%;
FLEXURAL STRENGTH PSI COMPRESSIVE STRENGTH PSI US. Patent Nov. 4, 1975' Sheet4 f8 3,917,781
8,000 COMPRESSIVE STRENGTH vs DAYS MOIST CURE 4,0 1 -7+ SACK MIX BEAR RIV. A00. 0
]I8+ SACK MIX BEAR RIV. A00. 0
' 7 l4 2| 2s 0 l I I I 0 IO 00 DAYS MOIST CURE (FOG) moo FLEXURAL STRENGTH vs DAYS MOIST CURE 500 I 0 SACK MIX -HEALDSBURG AGGJEI 11 0+ SACK MlX BEAR RIV. A00. 0 I 7 I4 2| 20 l l l I 0 I I I I DAYS MOIST CURE (FOG) SHEAR MOHR FIG 14 STRESS ENVELOPES/ FIG 15 a 'a I 6 2 2 NORIOIAL STRESS US. Patent Nov. 4, 1975 COMPRESSIVE STRENGTH PSI COMPRESSIVE LOAD (POUNDS) Sheet 5 of 8 3,917,781
|e,oo0 FIG 16 l3,000 I l I i l ANGLE OF THE MOHR ENVELOPE (DEGREES) m COMPRESSIVE STRENGTH l0,000 VS WATER CEMENT RATIO 8,000 I I 28 DAY El 11- 7 DAY O l l l l l l l WATER CEMENT RATIO GALS/SACK FlG 17 U.S. Patent NOV.4, 1975 Sheet7 of8 3,917,781
wmEw Qz w LO ZOrSmEFwE A V MNTW Imm=2 m WQM m mwe m OE US. Patent Nov. 4, 1975 Sheet 8 of8 3,917,781
OO O@ Omw Om O O m OW O m O m O mm T O 0? ON Om Ow Om O Om Ow 0Q 00? Z l7 (canvawoo) mm oE ALTERING THE PROPERTIES OF CONCRETE BY ALTERING THE QUALITY OR GEOMETRY OF THE INTERGRANULAR CONTACT OF FILLER MATERIALS CROSS-REFERENCE TO A RELATED APPLICATION This application is a continuation-in-part of application Ser. No. 886,556, filed Dec. 19, 1969, which was a continuation-in-part of application Ser. No. 792,370, filed Jan. 21, 1969, now abandoned.
BACKGROUND OF THE INVENTION This invention relates to improvements in concrete. It enables the making of stronger concrete and concrete that has more strength per weight than heretofore. lt also enables improvement of many other qualities and properties of the concrete.
The invention has three basic aspects. The first relates to the manner of formulation, compaction, and water addition; this aspect applies both when conventional and non-conventional materials are used, for it relates to any cementitious product which uses a' hydraulic cement as the binder for filler aggregate particles. The second aspect of the invention incorporates the first aspect and, in addition, relates to significant alteration of the normal properties of cementitious products through the addition in a novel manner of large quantities of materials able to impart desired properties. As an example, the normal density can be significantly reduced through the addition of a very low density material while imparting only minimum reduction in strength. A third aspect, which applies to both of these other aspects, through especially to the second one, is that of inhibiting segregation of the concrete mixture; i.e., it applies to preventing an optimum configuration and mixture from being dissipated by partial separation of some components of the mixture from others.
Concrete is normally mixed wet before casting, and forms a paste of the cement and water which coats all of the aggregate particles. When the wet mixture is cast into the mold, the aggregate particles are not in a direct contact with each other but are of necessity separated by a film or larger quantity of cement and/or water. This phenomenon of separation is quite independent of the degree of compaction of the mixture, both during and after casting, and we have found that it may impart a fundamental weakness to the concrete.
Another difficulty arises because wet mixes require the addition of considerably more water than what is actually required for hydration, the additional water being used to increase the flowability of the wet mix, so as to permit proper handling and filling of the mold or forms. As a result, the excess water eventually evaporates, leaving voids; hence, the concrete is left weaker than it would have been if the excess water had never been present. Also, large or small voids are left as a consequence of entrapping air during the placing process. Furthermore, shrinkage forces larger than desirable often exist, and they cause cracking and other structural weakening of the final concrete product.
A further difficulty heretofore has arisen because the quantities and sizes of aggregates chosen for a wet mix are based primarily on obtaining such workability in the wet mix as is commensurate with the quantity of. cement chosen for the desired strength. This basis of choice of aggregate sizes and quantities, coupled with the large quantity of water needed for workability, has often led to excessive segregation.
The present invention overcomes these problems by a novel dry-mix and dry-casting procedure.
The invention also relates to a novel use of lightweight aggregate constituents in a manner such that the strength of lightweight concrete is markedly improved.
SUMMARY OF THE INVENTION Among the objects of the invention are the provision of concrete having improved strength-to-weight ratio, improved compressive strength, improved tensile strength, improved surface hardness, reduced porosity, greater resistance to subsequent wetting, better asmolded surface appearance and uniformity, better texture both on the surface and in the body, ability to give more uniform color control when cast with pigments, better receptivity to paints, and better control of properties in general.
Briefly summarized, the method calls for dry mixing aggregates with cement (or a cement-filler mixture in which the particle size of the filler is of the same order of size as the cement) in such a manner that the volume of all the cement-size particles is approximately equal to the volume of void space which the aggregates would have if the cement-size particles were not present. A mold is filled withthe resultant dry mixture, the ingredients of which are so proportioned as to achieve, in consequence of a subsequent compaction step, a preferred geometry of particle contact. The quality and geometry of this contact (coupled with the properties of the particular filler materials, including aggregates) is a major determinant of the properties of the finished product. Then, after compaction and with no disturbance to the compacted structure of particle contact, water is injected and distributed by a self-metering process dependent upon capillary action-which itself is dependent upon the geometry of the particle contact. The water is introduced in a manner such that the water sweeps the air from the remaining voids and ejects that air which is directed into the atmosphere. The resultant wet, compact mixture is then cured in the mold or forms.
In lightweight concrete of this invention, segregation of the lighter components from the heavier components is inhibited, thereby solving a most pressing problem in the handling of materials and in the ordering of the structure of the dry material particles.
Other objects and advantages of the invention will appear from the following description of some preferred forms of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS In the drawings:
FIG. 1 is a diagrammatic view in elevation and in section of a mold containing a column of compacted aggregate, illustrating some aspects of an underlying theory of the invention.
FIG. 2 is a diagram showing a Mohr envelope for certain relationships between shear stress and normal stress.
FIG. 3 is'a similar diagram for another instance.
FIG. 4 is another similar diagram for yet another instance.
FIG. 5 is a diagrammatic representation of a loose 5050 pack of structurally strong and structurally weak particles.
FIG. A is a force diagram of a balanced loaded particle of FIG. 5, with the forces shown in equilibrium.
FIG. 6 is a similar diagram of a denser pack of the same types of particles.
FIG. 6A is a force diagram of an unbalanced loaded particle of FIG. 6.
FIG. 7 is a similar diagram of a pack of structurally strong particles around only one structurally weak particle.
FIG. 7A is a force diagram of non-symmetrical loading of the particles adjacent to the weak particle of FIG. 7.
FIG. 8 is a graph obtained from the data of Example 1 below, showing the relationship between the percent voids in compacted sand aggregate completely filled with compacted cement.
FIG. 9 is a graph of the compressive strength obtained per amount of cement in a mix of cement and phenolic microballoons.
FIG. 10 is a diagram showing how the beams of Example 5 are tested.
FIG. 11 is a view in cross-section of one of the beams tested according to FIG. 10.
FIG. 12 is a comparison of compressive strengths obtained from two basic mixtures of this invention with varying amounts of shrinkage induced with the aid of calcium chloride in the water added.
FIG. 13 is a graph where compressive strength has been plotted against days of moist cure for two basic mixes made according to the principles of this invention.
FIG. 14 is a graph of flexural strength plotted against days of moist cure for two basic mixes prepared according to the principles of this invention.
FIG. 15 is a Mohr envelope diagram comparing the shear stress per normal stress of two mixes differing only in cement content, the solid line envelope and circles representing a concrete having less cement per aggregate than the broken-line envelope and circles.
FIG. 16 is a graph wherein compressive load is compared with the angle of the Mohr envelope of failure for a series of sand-cement concretes differing only in the sands used.
FIG. 17 is a graph plotting compressive strength against the water-to-cement ratio for two different types of concrete both made according to the principles of this invention.
FIG. 18 is a graph showing the increase in compressive strength corresponding to increased shrinkage in concrete of the present invention which incorporates calcium chloride as shrinkage-inducing agent.
FIG. 19 is a graph showing the increase in flexural tensile strength corresponding to the increase in shrinkage in the same concrete as for FIG. 18.
FIG. 20 is a Mohr envelope diagram comparing the qualities of concrete with and without an agent that in creases shrinkage.
FIG. 21 is a chart of the distribution of sizes of two sands.
FIG. 22 is a graph for optimum blending of the two sand of FIG. 20.
DESCRIPTION OF BASIC THEORY AND OF PREFERRED EMBODIMENTS In the first aspect of the invention, graded fine and coarse aggregates, preferably with angular grains, are mixed together. If these aggregates were to be densely compacted, they would have a certain percentage of void space between particles. A quantity of cement calculated to be approximately equal to the volume ofthat void space when the dry cement is in a compacted dense array, is added. The dry aggregate is then mixed with the added cement, and the dry mixture is cast into the mold or forms, the casting being done in a manner minimizing segregation. The dry mixture is then compacted as much as possible in the mold or forms, as by a combination of vibration and tamping, so as to approach the volume which the well-compacted aggregate would have had without any cement being present; i.e., the cement completely fills all the voids between the aggregates. Water is then added by exposing a portion of the concrete to water, for instance, through small holes at the bottom of the mold. Capillary action is sufficient to draw the water throughout the concrete mix, sweeping the air in the minute capillary-size void spaces ahead of the water and eventually out of the mold, ejecting the air into the atmosphere. Additives, such as calcium chloride to induce shrinkage, pigments for color, and fibers for ductility, may be included. The concrete is then cured in the normal manner.
In the manufacture of concrete, both as done in prior art and as done in this invention, it is fundamental that non-uniformity of manufacture gives rise to nonuniformity of product, and hence, a product of poor quality. Segregation of aggregates is to be avoided, and uniform dispersion of all sizes is preferred. Quite independent of the art of manufacture is the fact that the purpose of prior-art mixing was to achieve this state of uniform dispersion of all sizes. The mechanics of segregation for the concrete of the present invention differ significantly from the mechanics of segregation of prior art wet-cast concrete and a knowledge of this difference is essential to a proper understanding and execution of this invention.
In prior-art wet-cast concrete, the matrix has the properties of a highly viscous fluid. If the mixture is sufficiently viscous, then the viscous drag forces, when mobilized, are sufficient to oppose both the settlement of the large heavy particles (buoyant force and viscous drag oppose the gravitational force) and the floatation of the lighter particles (gravitational force and viscous drag oppose the buoyant force). The greater the viscosity (tackiness), the greater the opportunity to mobilize the viscous drag forces. Too tacky a mixture results in the undesirable formation of rock pockets. Too wet a mixture gives rise to segregation, honeycomb, and an accumulation of laitance at the top. Vibration for the purposes of placement and compaction adds to the problem in two ways; a large part of the viscous drag potential may be consumed in resisting the dynamic forces of vibration, and when pore pressures (which accompany high-frequency dynamic forces) develop, theyact as additional floatation forces, which help in placement but harm in that they also promote segregation, honeycomb, and laitance accumulation.
For the concrete of this invention, the mechanism of segregation is strikingly different. Because all materials are handled dry, there are no floatation forces and no viscous forces. Frictional forces and gravitational forces are operative. Whereas in prior-art wet-cast concrete segregation is accompanied with the phenomenon of the heavier and larger particles settling to the bottom and the ligher and smaller particles rising to the top, segregation of a dry mixture is accompanied with the opposite phenomenon of the smaller particles settling to the bottom and the larger particles rising to the top. V
An example of dry-particle segregation is as follows: A transparent container of a well graded dry-sand, maximum size four mesh and a minimum size one hundred mesh, U.S. standard gauge, was filled to threequarters of its volumetric capacity. The container and the sand were subjected to manual shaking for a twominute period in such a manner so as to promote segregation. Inspection of the arrangement of sand in the container afterward showed a concentration of finer particles at the bottom, a concentration of coarser particles at the top, and a graduation of particle sizes of course to fine going from top to bottom.
An understanding of the mechanism of segregation of dry granular material aids proper execution of this invention. If the sand particles are acted upon with repeated motion in a vertical direction, the large and small particles alike respond to the forces causing such motion. The direction of motion is the same, but the duration of motion is different for particles of different mass. Considering only particles of the same material, when the direction of motion is upwards, the greater inertia of the larger particles causes the larger particles to continue upwards for a time duration longer than that of the smaller particles. The smaller particles began their fall sooner and fill the vacant spaces left by the larger particles. Repeated motion reinforces this mechanism.
The addition of dry cement, however, can interfere with this process of segregation of the aggregates. The cement is generally much finer than the ususal fine aggregates used. If compaction is by vibration, designed to excite only the aggregates (less than 4000 cycles per minute), the cement in the void space between aggregate particles provides significant mechanical impedance to the segregation of the aggregates. With cement, rather than air, filling the voids, the fine aggregates must either compact the cement or displace the cement upwards in order for the fine aggrergates to settle downwards. Friction, gravity, and the motions of the larger aggregates impede such upward displacement of cement. The addition of dry cement prior to vibration makes segregation of aggregates a much less likely event. It is only necessary to insure that there is cement rather than air in the voids. Proper and efficient prior mixing of all the dry ingredients, including the cement, will insure against the absence of cement in the aggregate voids.
When, however, the fine aggregate sizes are very small and approach the size of the particles of dry cement, the mechanics of mixing and vibrating are altered. Fine aggregates of this size are commercially available from quarry operations and are commonly known as rock flour or rock dust. When the smallest sizes of fine aggregates are large enough to be considered of sand size, e.g. that which is retained on'a 100 mesh sieve, adequate mixing is possible when all the dry aggregates and dry cement are placed in a mechanical mixer and mixed. When, however, the smallest sizes of fine aggregate are of the rock flour or rock dust size, simple mechanical mixing of all the materials is inadequate. For particles of this very small size, the forces of electrostatic attraction become significant when compared with the masses of the particles, and the beneficial effects of the gravitational forces (operating on the masses) are opposed by these forces of electrostatic attraction. Therefore it is difficult to cause separation of the flour or dust (in a usual mechanical mixing operation) to permit the uniform dispersal of cement be tween the particles of rock dust or rock flour. For this case proper and adequate mixing may be achieved if the cement and rock flour or rock dust are first premixed with a high energy rapid mechanical mixer, such as that known as a blender. An example of such a mixer is that used by cooks to beat, chop, or whip foods.
The same principle described above is to be applied to powdered pigments introduced to add color to concrete in the dry-cast process of this invention. The sizes of the particles of pigment are significantly smaller than the sizes of the particles of cement, and unless they are mixed so as to be uniformly dispersed, intensity of color will not be either constant or predictable. With uniform dispersion both constancy and predictibility are possible. Proper mixing of concrete ingredients therefore requires a high-energy premixing of the particles of pigment with the particles of cement.
During the compaction process, segregation is to be avoided. Densification of uniformly distributed material is the objective of the compaction process. When the dry cement in the voids has been sufficiently compacted so that the aggregate particles are in contact, each one with its'neighbors, the objective of compaction has been met. It is necessary as part of the mix design to insure that an excess of cement be avoided, for an excess of cement will preclude the possibliity of the aggregate particles forming the necessary contacts. Unlike prior-art concrete, increasing the amount of cement will not necessarily add to the strength, but, rather, increasing the amount of cement weakens the final product if aggregate contacts are precluded. The method of compaction is dependent upon the geometry and function of the finished product. Quite independent of the method of compaction are the principles, articulated above, which must be satisfied. Examples of compaction techniques satisfying these principles are stated below. i
i. Vertical pressure only.
This technique works well and is advised for use when the concrete product to be manufactured in thin, say 2 inches thickgor thinner, and when the molds are strong and competent to withstand heavy pressures. The greater the jsuruface area receiving the pressure, the greater the force required. This technique is satisfactory for the factory production of concrete tiles.
ii. Vertical pressure preceded by high frequency vibration.
This technique is a variation of (i) above and is useful for plates thicker than two inches. The vibration is used for better distribution of the material in the molds.
iii. Low-frequency vertical vibration only.
This technique works well when within the dry mixture there exist large aggregate particles, e.g. greater than three-eighths inch in diameter. The low-frequency vibration, less than cycles per minute, is designed to excite the masses of these larger aggregates which work like floating hammers within the compacting mixture. A drop table designed for low-frequency drops (vertical vibration) works well for factory production by this technique. The reason why high-frequency vibration, alone (greater than 4000 cycles per minute) is not satisfactory, is as follows: The larger the mass of the aggregate particle, the more slowly it responds to the excitation of the external vibration. If the vibration is very rapid, the vibration may reverse direction long before the inertia of the heavy mass has been overcome.
As a consequence energy of vibration may cause no excitation or response in the large particles. The small particles (small mass and small inertia) may, however, respond. They will then compact and may in fact segregate, and this is to be avoided. As a general principle, when vertical vibration alone is used, those frequencies which excite the large particles of aggregate are the desirable frequencies.
iv. Low-frequency vertical vibration and high-frequency horizontal vibration.
This technique is a variation of (iii) above. The purpose of the low-frequencyvertical vibration is exactly the same as described in (iii) above. The purpose of the high-frequency horizontal vibration is to help overcome the frictional constraints to the vertical compaction.
v. Vertical pressure and high-frequency horizontal vibration.
This technique is a variation of (iv) above. Vertical pressure (which can be thought of as the lowest possible frequency of low frequency vibration) is substituted for low-frequency vertical vibration. This alternate may at times be more convenient and is recommended for vertically long castings.
High-frequency horizontal vibration only.
This technique is a variation of (v) above that the vertical pressure is supplied by and limited to the weight of the dry materials cast in the mold or form. This technique with possible surcharging, is recommended for field application. Commercial vibrators can supplyhe horizontal vibration. It should be expected that this alternate will yield a finished product less dense than that of (v) above.
Coupled with the selection of compactive effort is the density of the finished product. Coupled with the density of the finished product is the proportioning of ingredients of the dry mixture. For instance, for optimum design, the cement content of alternate (vi) above should be greater than that of (v) above since the packing of the fine aggregates of (vi) is less dense that that of (v) and the void space to be fillled with cement is greater. In both cases of (v) and (vi) the cement content should never be greater than that which would permit the intimate particle contacts of the fine aggregates.
A very important feature of this invention is that the water which is introduced by capillary, gravitational, hydraulic pressure, or other forces must displace the air in the voids to be filled with water. Any mold or casting bed should be designed to avoid the trapping of air within the dry mixture to be infused with water. Trapped air inhibits the full penetration of water to the region of the entrapped air and tends to prevent the successful use of this invention. For instance, in the application to a slab, water should not be introduced simultaneously to both the top of the slab and the bottom of the slab; if it were so introduced, there would be too much risk that the cement hydrating at or near the top and bottom surfaces would seal against the exit of air from within. The infusing forces would be opposed by the developing air pressure within the slab until an equilibrium were established, and then no more water would infuse. If, however, the water is introduced at the bottom only, and if the top is exposed, then it is possible to sweep all the air upwards and out in advance of the infusing water. If the water is to be introduced at the top, then the bottom forms should be provided with 8 vents of sufficient frequency and capacity as to enable the orderly exit of air in advance of the infusing water.
If a section of concrete of this invention is so thick that there is a possibility that the infusing waters might cause the hydrating cement to seal the water courses prior to complete infusion, then preferred practice of this invention includes introducing the waters of infusion from with the dry mix in a manner timed to permit the orderly exit of air. The use of such internal conduits for the infusion from within as will enable infusion along the length of the conduit, such as for example, a stocking of sand or the use of B-X cable, is in accordance with the proper exercise of this invention under these circumstances, when internal infusion is warranted or is otherwise desired.
If the section of concrete of this invention does not warrant internal infusion, and if infusion is to be from without and from a formed surface, such as for example, the face of a wall or the bottom of a slab, then a course to distribute the water along the face of the form or mold should preferably be provided. Water distribution courses such as those provided by corrugated or screened contact surfaces are examples of recommended practice of this invention.
If water is to be distributed along a free top surface, such as for example, the top of a slab, then it is to be expected that, initially, the compacted top surface will be slightly disturbed by the exit of air upwards (air being lighter than water). This undesirable disturbance can be inhibited, according to this invention, by surcharging the top surface with a load such as, for example, sand. A surcharge load wil prevent the disturbance of the top surface of the concrete. As an alternative, the initially disturbed top surface may be recompacted while the mixture is plastic and the cement still has a significant portion of its hydration ahead of it. Very often, troweling of the top surface, after water has been infused from the top, provides the required amount of recompaction. A further alternative is to add water to the top surface sufficiently slowly to prevent the air near the surface from being trapped within the water and tending to bubble upwards rather than to exit downwards. Finally, these three alternatives can be coupled on any combination of any two or all three of them.
The proper compaction of the dry mix in the mold or forms prior to the addition of water causes the particles of cement to be contained within the void spaces that necessarily exist between the aggregate particles. Thus a separating film of either water or cement between the aggregate particles is avoided. Moreover, the dry mix aerates and flows almost like a fluid, enabling excellent mold definition even superior to a very wet mix. After compaction, the remaining void spaces are so small as to afford entry of only a small quantity of water, which comes in under the influence of capillary action. Very little more than the minimum required for hydration is used, and the process is self-metering.
A feature in some forms of the invention is that the quantity of each size of aggregate used in formulating a dry-cast concrete of this invention is chosen so that each size range fits into the void spaces between the particles of the next larger size. Therefore, with proper proportioning of quantities, the most compact state of the dry-cast mix can only exist with no segregation, and the very act of compaction tends to create the correct unsegregated distribution of particles.
' manner Dry mixes formulated and poured into molds in the stated above have yielded compressive strengths far in excess of what might be expected from the more usual wet mix formulations of the same cement content and same manner of cure. For example, laboratory test results shown in FIG. 13 were obtained from cement-aggregate mixes that included no chemical additives and there was no accelerated cure. This figure shows that 28-day compressive strengths of approximately 10,000 psi have been obtained for a nominal 7-sack mix, and strengths of about 11,000 psi for a nominal 8-sack mix. Twenty-eight day compressive strengths 12,000 psi have been obtained for 10.5 sacks of cement per cubic yard, instead of themore usual 6,000 psi for a wet mix of the same formulation an improvement of about 100 percent. Other dry mixes of this invention have given compressive strengths of 6,000 psi for 5.5 sacks of cement per cubic yard instead of the more usual 2,500 psi for the wet mix; and 5,000 psi for sacks of cement per cubic yard instead of the more usual 2,000 psi for a wet mix. The wet mixes described above are those formulated for 3 to 4 inch slump. This marked improvement in compressive stength is only one of the advantages of this invention. A considerable increase in tensile strengths also results. For the cement of this invention having a 12,000 psi compressive strength, a tensile strength in excess of 1,000 psi was obtained.
Particular note should be taken of the very high early strengths of concrete formulated in the manner of this invention. FIG. 13 also shows that compressive strengths in excess of 4,000 psi have been achieved after 1 day of non-accelerated curing for both the 7- sack and 8-sack mixes of this example. These strengths are far in excess of those attainable by the usual wet mix formulations of similar composition and subjected to similar curing.
A considerable increase in the flexural tensile strength occurs when this invention is employed to manufacture concrete. Hitherto unattainable flexural tensile strengths in excess of 1,000 psi fora nominal 6- sack mix and strenghs in excess of 1,400 psi for a nominal 8-sack mix have been obtained, without using any chemical additives. FIG. 14 shows that remarkable lday flexural tensile strenghts of approximately 600 psi have been obtained without any accelerated cure of the beam specimens. This example serves to illustrate that a distinct quality of high flexural tensile strength, which is not obtainable by similar formulations of the more usual wet mix methods, is obtained by the dry-cast processes of this invention.
Another noteworthy property is that the total included porosity for the dry-cast concrete of this invention is in the order of only 3.6 percent, which is almost an order of magnitude less than the porosity expected for conventional wet mixes. Consequently, the dry-cast concrete of this invention is virtually waterproof, approaching the permeability of a fired ceramic, which has 2 percent porosity. This quality is important for waterproofing, and it also enables painting without having to seal the concrete first and it imparts the ability to achieve a high-glass finish with one coat of paint.
The as-cast molded surface of dry-cast concrete of this invention is smooth, and, when formed in a proper mold surface, the as-cast surface gives a virtually polished appearance, completely free of visible honeycombing and air bubbles, thereby distinguishing it'from the undesirable honeycombed surface of a stiff wet 10 mix. This surface quality is of particular interest when it is further recognized that immediately upon wetting, the dry-cast concrete of this invention has considerably greater strength than any possible wet mix, no matter how low the slump of the wet mix.
Since water is not added to the dry-cast concrete of this invention until the concrete mix is already placed in the mold and since no movement of the wet material is required, considerably more rapidly curing mixes, such as high-early-strength cement and solutions of calcium chloride in the water, (e.g., 8% solutions) may be used with excellent results. Five-foot-square panels, varying in thickness from three-fourths of an inch to 2 inches with an average thickness of 1 inch, have been cast by this technique and, without heating or steam curing, were stripped from their forms and stood upright within ten hours of casting a time that by no means represents the lower time limit, for the time could have been substantially reduced.
Another very important result of the invention is that the dry-cast concrete of this invention is considerably more dimensionally stable than the conventional wetcast concrete. For example, a normal wet-mix concrete shrinks considerably during cure, so that a cold joint cannot bond well. In fact, one normally expects such a cold joint to crack. In contrast, thicknesses of concrete as little as a quarter of an inch have been patched with dry-cast concrete of this invention compacted in place against partially cured concrete, and the resultant structure appears to have suffered no reduction in strength through the cold joint. No crack or other characteristic readily distinguishes the patched areas.
The substantial reduction of both shrinkage and creep in the present invention results from the fact that substantially all of the aggregate particles are initially in contact with each other and upon the addition of water cannot be moved closer together by shrinkage in the cement or in the mix. The extreme lack of porosity and the stable particle arrangement enforced by intergranular contact prevents severe dimensional changes from occurring upon either the wetting or the drying of the concrete, and there is virtually no warpage in curing.
A theory which may explain these remarkable characteristics is offered, though the invention is not to be limited by such a theory. The results are obtained, whatever the correct theoretical explanation.
Consider an arrangement of granular particles, such as sand, in which every particle is in contact with one or more adjacent particles and in which the entire arrangement of particles is constrained so as to inhibit movement outward from the mass of the particles in contact. This may be accomplished, for example, by placing the sand in an infinitely rigid constraining cylinder 10, as shown in FIG. 1. Furthermore, consider that the column 11 of sand has been compacted to its most dense array so that further compaction without fracture of the particles is not possible. Such a column 11 of sand will then support a surcharge loading 12 without inelastic deformation, provided that there is no fracture of the particles in contact. In this case, the energy contributed by the surcharge loading 12 is stored internally and is available for use to effect complete elastic rebound when the surcharge loading 12 is removed.
The column 1 1 of particles in contact has some properties of a fluid, as can be seen by imagining the cylinder walls 10 suddenly removed. The column 11 of sand would then flow out from under the surcharge loading l2. Without lateral restraint, the column 11 of sand obviously has negligible strength. Thus, the strength of the column 11 of sand depends upon the quality of the constraining medium, in addition to the quality of the sand. If the cylinder walls were to be thin and flexible, then the maximum surcharge loading 12 causing failure would be less than if the walls 10 were thick and rigid. The column 11 of sand can operate well as a structure if and only if the outward movement (flow) of the mass of particles is inhibited, thereby enabling transfer of load through the particles in contact.
The preceding description of the column of sand is compatible with the Mohr-Coulomb strength theory, first expounded by Coulomb in the year 1776. The Mohr-Coulomb strength theory as applied to granular soils takes into account that a certain relationship exists between normal stresses and shear stresses, such that when a permissible combination of such stresses is exceeded, failure occurs. This relationship is given by a Mohr envelope, as shown in FIG. 2. By plotting the confining pressure, the minimum principal stress a, on the axis of principal stress, and then drawing a circle tangent to the Mohr envelope passing through this point and extending in the direction of increasing stress, the maximum principal stress 0- will occur where the circle crosses the axis of normal stress. The deviator stress 0 -0 is the load carried by the soil structure.
Suppose the confining pressure (r is increased to 0- The tangent circle is drawn and a is noted on the axis of normal stress. Note that an increase in confining pressure enables an increase in load-carrying capacity. That is, the deviator stress for the second case try-0', is greater than the deviator stress for the first case A second way of increasing deviator stress, i.e., the load carrying capacity of the sand structure, is to increase the angle a of the Mohr envelope. This is shown in FIG. 3, where o' "--a is greater than 0 -0- for the same confining pressure, 0-
A third way of increasing the load-carrying capacity of the sand structure is to build into the structure a shear capacity quite independent of the intergranular contact. This shear strength is marked C on the ordinate of shear stress, an FIG. 4 shows that for the same confining pressure and slope of the Mohr envelope, the deviator stress tu -0' is greater for that material with initial shear capacity than is the deviator stress 0 -0- for the same material but without the initial shear capacity.
The concrete material of this invention is composed of a compacted structure of granular material (which for the moment may be called sand, though applicable to all granular media) which depends heavily on the character and quality of the intergranular contact between particles. The cementitious material (Portland cement is used as an example) serves to provide both a constraining force (confining pressure) and an initial shear capacity independent of the intergranular contact.
When the sand and cement are mixed dry, the cement (being many orders of size smaller than the sand) will most conveniently and most naturally fill the voids between the particles of sand, when the mixture is compacted to a dense array of the sand. The geometry of contact then taken is preserved after water is added for hydration of the cement, for the self-metering aspect of feeding the water by capillary attraction provides only 12 that water which is needed to fill the remaining void spaces between the cement particles and this turns out to be little more than that required for hydration of the cement. The cement during its hydration period first expands largely filling its own void spaces, bonds itself to the particles of sand, and then contracts during the latter part of the hydration period. The contact force (or pressure) between the sand particles is increased due to this shrinkage of the bonded cement and to the disposition of the sand to maintain its geometry due to its already dense array. This prestressing of the sand structure provides the confining pressure indicated by 0- in the previous examples of the Mohr strength theory. The very presence of the cement binder itself also provides the initial shear capacity C noted by the ordinate of shear stress in FIG. 4.
For the optimum execution of this invention, there should be the proper relationship between the quantities of sand and cement. If the mixture is such that the voids between the sand particles are not completely filled with cement, then opportunity for the largest possible initial shear strength C and opportunity for the largest possible pre-stress of the sand structure has been lost. Such a condition of a lean mix (i.e., lean in cement content) precludes the optimization of the quality of high compressive strength, though it may be desirable for some other reason and though great improvements over conventional wet mixes are still obtained. It is also true that too rich a mix (i.e., rich in cement), which describes that condition where more cement is used than that needed to fill the voids, causes the particles of sand to separate and thereby destroys the effectiveness of the particle contact so heavily relied upon. For the usual case where the strength quality of the sand is superior to that of cement then both cement-to-cement contact and sand-to-cement contact are less effective than sand-to-sand contacts.
Blending of sands can result in a denser state than that which results from the use of any of the primary sands, and the quality of concrete improves with increasing density, or compactness.
FIG. 21 charts the distribution of sizes of two different sands which are commercially available in Israel: Sand N (Nizzanim), a rounded beach sand of high quality quartz, and Sand T (Tirat Yehuda), a low quality quarry sand of calcarious rock, very angular in particle shape. Note that in the distribution of grain sizes of FIG. 21, these two sands complement each other, in that a high concentration of sizes of one sand corresponds to a low concentration of the same sizes of the other.-lf these sands are combined in varying proportions and subjected to compaction, similar in character to that which is used in the manufacture of the concrete of this invention, those proportions which give the densest array should be selected for use.
FIG. 22 shows that a range between -60 percent T and 30-40 percent N gives the densest array. The specific gravities of the particles are the same for each sand, so that a simple dry weight of a unit of the combined sands reveals the densest geometric array.
Thus, concrete test specimens were made, and the results of applying the same compactive effort for all cases are shown below:
Compact- Cement Days Sand ness Compressive Content wet Proportions Voids strength cure %N %T in Kg/cm Kg/m sands (l p.s.i. (275 kg/m =0.07 5 sacks/yd) kg/cm) 285 3 I00 0 40.8 259 282 3 6O 40 36.3 319 280 8 100 0 4L9 349 270 8 60 40 39.1 378 The above tabulation demonstrates that even though 40 percent of the more competent Sand N is replaced by the less competent Sand T, the strength of the blended mix is improved. This improvement in strength is due to improvement in the character and quality of the intergranular contact due to an increase in the density and compactness. Due to the increase in the compactness and angularity of the sand strata, a marked increase in strength resulted, in spite of the fact that, in part, a weaker sand was substituted for a stronger sand.
Another study using Sand N (Nizzanim) and coarse aggregate H (l-Iar-Tuv) with varying cement contents and identical compactive efforts was made, with the following results:
Thus, an excess of cement can weaken the quality of interparticle contact.
As evidence that the sand particles are, in fact, in contact and that, as predicated in theory, by mobilizing these contact forces, added strength is introduced, the following is offered:
For the manufacture of floor tiles of concrete ingredients applicable to the process of this invention were placed in molds cm. 20cm. in horizontal area X l.5cm. deep; then compaction was accomplished by vibration and by applying pressure to the previously uncovered top. After the removal of the pressure, the interparticle contact was so complete, that it was possible, and is recommended for practice, to remove the bottom of the mold so that the water when applied to the top could infuse by driving the air out the bottom, which, being open, permits the exit of air. When water was applied to the top without the removal of the bottom form, air bubbles always formed within the 1.5 cm. thickness, preventing full infusion and also causing cracking when attempts were made to force water to all points by reapplying pressure. A square tile whose side was more than thirteen times its thickness was now held at the edges along its thickness, by friction only and this tile not only retained its stability before and after wetting, when nothing else than interparticle friction could be holding it before wetting, but also deflections were negligible. Seconds later, the tile was forcibly ejected from the mold, tilted to a vertical position, and removed by hand to a rack on which it was placed vertically.
In the usual wet process for the manufacture of concrete, the particles of sand are forced apart by the watercement phase in order to provide the workability required for the wet process. The strength advantage of intergranular contact of the sand is not inherent in the wet process and is not likely to be obtained. In addition, in the wet process, shrinkage of the cement due to hydration causes shrinkage of the concrete and dimensional instability. Shrinkage of the concrete in the dry process is virtually impossible because of the predetermined keyed structure of intergranular contacts.
As shown in FIG. 3, increasing the angle of the Mohr envelope increases the load-carrying capacity of the sand structure. As is well known in the field of soil mechanics, an increase in the angularity of the particles and the use of awell-graded mixture (i.e., one with many sizes of appropriate quantities), other things being equal, increases the slope of the Mohr envelope. It is reasonable to expect that for very dense angular well-graded sands the slope of the Mohr envelope will approach 45. This angle is preserved in the dry process of this invention, since the structure does not change its geometry. The literature suggests that for concrete of the wet process of casting concrete, a slope of only 20 might reasonably be expected. This unfavorably low angle is due to the unquestionably poor state of intergranular contact] The idealized Mohr envelope of failure may again be used to provide a theoretical explanation as to why large tensile "strengths occur as a consequence of this invention and why these tensile strengths increase with increasing cement content, provided that the cement content is not so great that it would force separation of the sand particles in contact, thereby removing the strength derived from the intergranular contact of the sand particles, which is represented by the Mohr envelope of failure.
The solid lines of FIG. 15 describe an idealized Mohr envelope of failure and the Mohr circles which are compatible with this envelope. The broken lines of FIG. 15 describe a similar condition of compacted aggregates, with an increase of cement content being the only difference; Note that increasing the cement content while the other variables are held constant causes an increase -th'e.initial shear capacity (the intercept on the shear stress axis) and in the shrinkage forces during the latter stages of the hydration. If the compacted aggregate particles are in intimate contact, then these additional shrinkage forces serve to further increase the cori'fining pressure from 0 to 0,. It is noted that this would cause an increase in the compressive strength (0, 0,), compare with (0- 0-,), and also, a rather significant increase in the tensilestrength (0- 0-,), compare with (0 -0 In the prior-art wet process for making concrete, it is not possible to get the tensile strength attainable by this invention, because the shrinkage of the hydrating cement is not effectively resisted by the aggregates (which are not in intimate contact, but rather are separated by a coating of cement and water). As a consequence, little confining pressure (0,) is set up by the shrinkage forces. Study of FIG. 15 shows that as 0', approaches zero, not only does the compressive strength (0' 0-,) reduce, but so does the tensile strength (a 0' and rather significantly at that. In the prior-art wet process, this condition of the confining pressure approaching zero exists, seriously inhibiting the development of load-sustaining tensile stresses.
In summary, this invention appears to provide a wellordered, pre-determined and compacted structure of aggregate with voids filled with cement and with water fed by capillary attraction and/or hydraulic pressure after the placement of the other materials, and the resultant concrete can sustain remarkably high loads and possesses dimensional stability not known to the process of wet manufacture. In, addition, the Mohr- Coulomb strength theory is applicable, so that predictions of quality of performance can be made, based upon laboratory study of the sand or other aggregate to be used in the basic structure. None of the above precludes the use of large aggregates to be added in the fine aggregate or to the sandcement mixture discussed above.
The strength of the concrete cannot exceed the strength of the aggregate particles and, therefore, higher strength requires sound aggregate with adequate compressive strength. An angular shape, such as tends to result from crushing larger particles, leads to greater strength than a rounded shape occurring in older sand and rock where sharp points have been worn off. Furthermore, within a given stratum, there should be a variation in size, for the strength theory shows that such variation increases the slope of the Mohr envelope, thereby leading to greater strengths. Basically, greater angularity and greater size variation causes greater interlocking when the dry mixture is compacted, and hence greater resistance to deformation and movement.
.For proper proportioning of aggregates and cement, it is necessary to create a compacted void volume in the aggregate structure approximately equal to the quantity of cement desired in the final mix. Adding additional cement requires proportioning aggregates so that the compacted void volume is adjusted to be large enough so as to properly receive the additional cement and yet preserve the integrity of the intergranular contact. In such a case it is possible to increase the strength of the final concrete product. However, adding that much additional cement which is in excess of the capacity of any compacted aggregate void volume to receive this additional cement, decreases the strength--for there is an excess of cement over the void spaces to be filled and the integrity of the intergranular contact of the aggregate structure is destroyed. For any particular distribution of aggregate sizes and shapes, there is an optimum cement contentthat is densest in a compacted dry state and fills the aggregate voids. The optimum compacted aggregate structure lies in between the densest possible arrangement and the loosest possible arrangement.
Consider a container of dry cement subjected to a compactive effort composed of tamping, vibration or other mechanical means. For this cement there is a particular lower bound of volume to which any given quantity of cement will compact. This lower bound is a stable figure which is a consequence of the equilibrium established between (1 the tendency of the particles to draw closer due to the mechanical effort expended during the compaction process, (2) the tendency of the particles to spread apart due to the air pressures in the voids (aeration) during the compaction process, and (3) the geometric and frictional impedence to both contraction and expansion in the neighborhood of this state of equilibrium. This particular equilibrium point will be called herein the most dense array, and it lies between the theoretical dense pack and theoretical 16 loose pack" arrays of spheres in contact. For example, the specific gravity of the most dense array of Type I cement was found to be in the neighborhood of 1.8.
The quantity of cement which fills the voids of the fine aggregate so that the cement is in the most dense array is the quantity which aids in maximizing the strength of the finished product. When designing a mix for maximum strength, it is an object of this invention to approach this most dense array for the cement in the voids of the fine aggregate. It is compatible with the strength theory of this invention that this will develop maximum shear intercept (FIG. 4), and maximum confining pressure, a of the fine aggregates due to shrinkage of the hydrating cement after the infusion of water. Friction at the interface between the fine aggregates and the cement will act to impede the realization of the most dense pack of the cement during compaction. It therefore becomes desirable to minimize this frictional impedence.
This may be accomplished by designing mix proportions such that the volume of voids in the fine aggregate which is filled with cement is large enough to permit compaction of the dry cement to a point approaching its most dense array and yet preserve the integrity of the particle contact of the fine aggregates. Reflecting again on the strength theory of this invention, enlarging the void space of the fine aggregates will flatten the slope of the Mohr envelope, thereby reducing the load carrying capacity. However, this disadvantage may be partially or fully offset by increasing the opportunity for approaching the most dense pack of the dry cement, thereby increasing the load carrying capacity. The amount of increase of void spaces in the fine aggregate which will produce the optimum strength (load carrying capacity) depends upon, among other variables, the compactive effort, the distribution of sizes of fine aggregate, the surface texture of the aggregate, and the type of cement. It has been found that when a washed and dried Del Monte sand was used with the following proportions:
26.5 No. 16 mesh 26.5 No. 20 mesh 26.5 No. 30 mesh that 34.5 No. of Type I cement provided optimum voids in the structure of fine aggregates. This volume of voids is approximately 25 percent greater than the volume of the voids when the sand is compacted to its dense pack state and is less than the volume of voids in the sand where the sand is in its loose pack state.
EXAMPLE 1 Specific gravity of the well-graded compacted sand l.65 aggregate in its most dense arrangement Specific gravity of the well-graded compacted sand 1.40 aggregate in its most loose arrangement Solid specific gravity of the sand aggregate Three dry mixtures were then made, as follows:
1 part cement. 3 parts sand (by weight) By calculation, Mixture A was found to have the following volume relationships:
= 0.32 units 0.61 units Volume of voids of compacted loose arrangement Volume of compacted dry cement Thus, in Mixture A there is a substantial excess of cement over the void space available to receive this cement. The ratio of the volume of the cement to the total volume was 66 percent. This mixture was tested and found to have a compressive (28 day) strength of only 2,800 psi.
By calculation, Mixture B was found to have the following volume relationships:
Volume of voids of compacted sand (40%) 0.61 units Specific gravity of compacted dry cement 1.58 Specific gravity of compacted sand 1.49
Volume of voids of compacted sand (34%) 0.62 units Specific gravity of compacted dry cement 1.37 Specific gravity of the compacted sand 1.65
The specific gravity of the sand in Mixture C lies at the densest arrangement, and all the voids are essentially filled with compacted dry cement. The compressive strength (28 day) was found by test to be 5,200 psi.
Example 1 thus indicates that a gradual increase in strength is to be expected with increase in cement content, as long as the sand aggregate forms a competent compacted structure. When the loosest possible arrangement of sand structure is no longer possible and integranular contact is compromised, a decrease in strength follows even with an increase in cement content. These results show consistency with the strength theory of this invention. For, when the integrity of the intergranular contacts of the aggregates is compromised, the effect is to markedly decrease the slope of the Mohr failure envelope and, as previously discussed, to decrease the deviator stress, which is the measure of the failure load. By increasing the void volume and yet keeping the integrity of the intergranular contact of the sand, although the slope of the Mohr envelope may decrease slightly, the effect of the added cement at increased density is to increase the initial shear stress intercept (shown in FIG. 4 as C on the ordinate axis) as well as to increase the confining pressure of the aggregates in contact due to increased shrinkage forces of the cement gel. Both these effects of the additional cement add more to the deviator stress (the measure of failure load) than the slightly flatter slope of the failure envelope detracts from the deviator stress. The net result is to show an increase in load-carrying capacity,
18 which is indicated graphically in FIG. 8, a plot of the results of this example.
EXAMPLE 2 To show that increasing the quality of intergranular contact increases the strength of the resulting concrete product.
The literature of soil mechanics shows that sand aggregates of uniform size will not have as steep a slope for the Mohr failure envelope as will a well-graded mixture of the same sand, other things being equal. Therefore, because the well-graded mixture is known to have a higher quality of intergranular contact, as a soil, then it is predicted by the theory discussed above that the well-graded mixture will produce a higher-strength concrete. This is borne out by the information supplied in the table below. The source of the sand aggregate was the same in both cases. The compacted dry density of the cement was constant for the tests of each case.
Compressive Strength (28 day) psi The following example is offered to illustrate that the compressive strength of the concrete of this invention is improved with increasing slope of the Mohr envelope of failure (a measure of the quality of intergranular contact). Tests were conducted on three different types of sands to determine the Mohr envelope of failure under similar conditions of compaction. All other variables such as quantity and type of cement, size and shape of sample, compaction of dry material, technique of curing, etc., were kept constant, so as to effectively isolate the variable of interest the slope of the Mohr envelope of failure of the sand phase. The three sands used were an Ottawa 20/30, an Amador 70, and a well graded Silica. The results are charted on FIG. 16 and clearly demonstrate the principle that as the slope of the Mohr envelope increases so does the compressive load-carrying capability of the concrete of this invention.
EXAMPLE 4 Specific Gravity of dry compacted cement (identical Compressive cement except for Strength in Mixture compaction) psi (7 day) F 1.16 2,260 G 1.37 3,140 H 1.48 4,200 l 1.54 5,360
The specific gravity of the dry compacted cement is related to the amount of water that can be absorbed by capillary action. Excluding absorption of water by the aggregates, the amount of water which may be admitted by this dry cast and subsequent wetting process can be no more than the space between the particles of cement. The greater the specific gravity of dry compacted cement, the smaller the volume of void space between the cement particles, and hence, the smaller amount of water admitted. Consequently, the denser the compacted dry cement, the smaller the quantity of water admitted and, therefore, the lower the water-cement ratio. The watercement ratio is a significant parameter which relates to the strength of concrete. The lower the water-cement ratio, the greater the strength, other things being equal. Whereas, in the usual process of manufacturing wet mix concrete, watercement ratios are rarely less than 4 gallons per sack of cement and this ratio is achieved only by reducing workability, surface smoothness and form definition the concrete of this invention can achieve water-cement ratios less than 3 gallons per sack of cement without such negative factors. This low water-cement ratio does not introduce the likelihood of marred surfaces due to trapped air, as it does in the wet process. FIG. 17 illustrates the improvement in compressive strength of cement-fine aggregate-coarse aggregate concrete of this invention with reduction in water-cement ratio. The
low water-cement ratios obtained and the related high compressive strengths are not possible with prior-art concretes of the same material formulation. In addition these qualities are obtained with the same excellent form or mold definition discussed in other parts of this specification.
That which is true for the proportioning of cement to sand (fine aggregate) is likewise true for the proportioning of fine aggregates to coarse aggregates (rock). If the cement sizes are completely telescoped within the sand-size voidsand completely fill these voids, and if the sand sizes are completely telescoped within rocksize voids and completely fill these voids, then the quantities which result can be adjusted to achieve proportions for maximum strength. That quantity of rock, which when compacted to the state of interrock contact but which admits large quantities of wellcompacted mortar which completely fill the voids but do not over-fill them approaches the optimum quantity. Care however, must be taken so that the formation of inter-rock contact does not preclude the likelihood of sufficient compaction of the fine aggregate strata. If it did, and if inter-particle contact of fine aggregates were not complete, then a loss of strength should be expected. It is advised that the compacted state which includes inter-rock contact be considered a theoretical objective of optimum design. A practical objective should be to include a lesser quantity of coarse aggregate (lesser than that required for complete inter-rock contact) in order to assure optimum contact of the fine aggregates.
In the dry mixture of aggregate and cement, if the aggregate particles are small enough, the viscous drag of forces on them predominates over dynamic or buoyancy forces. During normal mixing and handling, if the aggregate and cement are reasonably close to the same density there is no segregation except in the immediate vicinity of the free surface.
The free surface of the dry mixture has an effect not unlike surface tension of a liquid. An aggregate particle 20 near the surface, when some form of agitation exists, has a certain probability of being moved to the top of the free surface. Because of the analogous apparent surface tension, the probability of the particle reentering the surface is less than the probability of its leaving the mix. Therefore, for any given mixture of sizes of aggregate, there is a sort of scum of free particles of aggregate created on the surface. The total number of particles of a given size on the surface remains nearly fixed and is equal to the quantity required to overcome the lower probability of the particle re-entering, and thus a state of equilibrium exists. Since this is a surface phenomenon only, then the mix immediately below the surface lacks somewhat that amount of aggregate resting on the surface. Therefore, the act of surface compaction tends to restore the mixture to the proper state.
The dry mix is, therefore, quite free from segregation problems so long as the surface scum is not removed. If the material is allowed to flow down a long open chute, whose angle of fall is steeper than the angle of repose of the aggregate on the surface, the aggregate on the surface will outrun the mixture and the true segregation tends to occur. The same effect occurs from material being poured from a spout and allowed to build up, forming a cone, sides of which have an angle greater than the angle of repose.
These problems can be overcome by limiting the chute angle to no greater than approximately 30 above horizontal, the precise angle depending on the angularity and size of particle. For angles greater than this, a closed conduit made to run full, having no free-surface, is also an adequate solution. In casting into the mold, care must be taken not to build up a cone or mound with sides that are too steep. If a storage vessel is to be used, then it should be filled from one side at the top and removed from the opposite side at the bottom, or it should be slightly agitated or some other provision made, such as a desegregation cone in its bottom.
Thus, though some segregation is possible, proper handling can and does largely eliminate it. In any case, the presence of the dry cement drastically reduces the degree of segregation that would be found in handling the dry aggregate by itself. 7
In the design of a mix for the dry-cast technique, two competing alternatives must be considered and evaluated for maximum efficiency. One is a mix whose sizes are such that, during the act of compaction, the cement-size particles are telescoped completely within the void spaces of the sand-size aggregate and the sandsize particles are slightly greater than that minimum number which will cause them to be telescoped completely within the void spaces of the rock size aggregate. This will be relatively more costly for compaction and relatively less costly for cement than a second mix designed for transportation and placement, without segregation except for surface scum. This second mix has a greater proportion of fine materials (including cement)" and a lesser proportion of coarse materials.
EXAMPLE 5 Mixture J Telescoping sizes moderate compaction required. A mixture of 1 part cement Type II, 2 parts sand and 4 parts rock (5 1 inches) made according to this invention, developed a compressive strength (28 day) of 6,100 psi. The cement content was 6.4 sacks per cubic yard.
Mixture K Transportable without segregation light compaction required. A mixture of 1 part cement Type I, 2.3 parts sand, and 2.77 parts rock (A-5i; inch), made by the process of this invention developed a compressive strength (28 day) of 5,000 psi. The cement content was 7.4 sacks per cubic yard.
In Mixture J, the cost of the moderate compaction would be offset by the savings in cement. In Mixture K, the cost of additional cement would be offset by the savings in compaction cost.
The capillary forces are considerable, and the water, once exposed to a surface of the concrete in its dry form, can be raised a considerable height. With the entry of the water, a potential difference is developed, which forces the air to the free surface of the dry mix. There is, therefore, considerable latitude in the manner in which the water is added, so long as the hydration rate is not accelerated so rapidly through the use of high early cement, calcium chloride, hot water, etc., that the capillary passages are shut off by the expanding cement gel before the addition of the water is complete. If the water is added to the upper surface, this surface may have to be troweled or brushed subsequently, for appearance sake. If the water is added through internal tubing, the surface can be brought to a smooth troweled finish prior to the addition of water.
The process of adding water is self-metering, in that the void spaces can hold only the right amount of water. So long as the water is not added with sufficient turbulence to form a wet mixing action, the process may be speeded up by raising the pressure on the entering water. This pressure must, however, be sufficiently low so as not to disturb the pre-formed intergranular contact of the compacted dry-cast materials. The process may, of course, also be speeded up by increasing the number and area of entry points for the water.
The self-metering aspect means that no consideration need be given the porosity or hydrophilic nature of the aggregate. This, coupled with the fact that the water is not added until after the concrete is placed, gives considerable additional advantage to the process beyond the achievement of the better properties.
The second aspect of this invention is of use where it is desired:
l. to maximize or minimize the strength of a given or specific formulation of component materials, or
2. to minimize or maximize the segregation of the component materials in a given or specific formulation, particularly where there is a difference in densities, or
3. to maximize a quantity of one or more of the component materials weaker in strength than the other components, while a minimum decrease in strength of the resultant composite product is desired, or
4. to significantly alter one or more of the properties or qualities of a composite cementitious material, or
5. to minimize the quantity of one or more of the strong component materials or the cement with a minimum decrease in strength.
The essence of this aspect of the invention is to form a predetermined ordered structure within and between the various size ranges of particles or strata so as to maximize or minimize, whichever is desired, some property, such as strength. This is accomplished through the selection of particle sizes, the selection of specific quantities and proportion of these particle sizes, and the selection of component materials.
Thus, if the segregation of a very lightweight component is to be prevented, then, in accordance with the 22 practice of this invention, one of the following may be done:
a. The lightweight particles may be made small enough so that the forces of viscous drag exceed the apparent buoyancy forces in either a wet or dry mix.
b. The size and quantity of lightweight particles may be chosen so that they fit completely within the void spaces between particles of a larger and heavy kind, and the quantity and proportion of the larger particle size may be chosen so that the larger particles touch each other in the completed mix. This means that the lighter particles would have to shove aside the heavy particles in order to segregate an unlikely event.
The quantities of smaller particles determine the spacing of the larger particles. If that spacing is chosen such that, in the compacted mix, wet or dry, the particles all touch; then a compressive force must pass through them. If these particles are either the strongest or weakest component materials, the strength is, respectively, greatest or least.
A number of component materials have been added to concrete such as sawdust, phenolic and other microballoons, foamed silica, foamed polystyrene, and foamed ceramics. The theory of the invention has been tested with each of these materials and has been borne out by all themixes.
Densities as low as pounds per cubic foot, depending on the lightness of the particles added, have been obtained with compressive strengths of approximately 4,000 psi. Youngs modulus has been reduced by an order of magnitude. Shrinkage has been both increased and decreased, according to what is desired. Densities as low as 50 pounds per cubic foot, with 1,500 psi compressive strength have been achieved. Porosity has been both increased and decreased, according to what may be desired. Machinability has been increased to a point where the concrete can be cut with a carpenters saw or threaded on a lathe with the normal high-speed tool bit shaped for the cutting of steel.
Many other properties may be drastically changed by practicing this phase of the invention. For example, the nuclear shielding properties may be improved. As another example, brighter colors may be obtained through the addition of larger quantities of color pigment. In another instance, forty percent of the cement specified in a given mix has been omitted while retaining 80 percent of the original compressive strength. In other instances, particles in densities as low as one-half pound per cubic" foot have been added and contained in the mix without segregation.
Concrete normally consists of four components: coarse aggregate, fine aggregate or sand, cement particles, and water. In the usual wet mix, the sand is at least partially contained within the void spaces of the coarse aggregate. The cement particles, are at least partially contained within the void spaces of the sand. The excess water is .at least partially contained within the void spaces of the cement. The spacing between the particles of a particular component is defined by the total volume assumed by the smaller-particle components. In the particular case where there is no smaller-particle component, the component will compact to a point where all of its particles are touching. Depending on the amount of compaction, the form of structure achieved will vary. Perfect spheres, all of the same size, can exist in structurally stable arrays where there can be as much as 48 percent void space and as little as 28 percent void space. These are referred to herein as